1. Field of the Invention
The present invention relates generally to an apparatus and a process for supplying a chemical to processing units for producing semiconductor devices, and, more particularly to a process and apparatus for supplying a chemical prepared by diluting and mixing stock solutions to semiconductor production-processing units.
2. Description of the Related Art
Various types of chemical supplying apparatus are employed in the production of semiconductor devices. The chemicals supplying apparatus supply chemicals, prepared by diluting stock solutions with pure water or by mixing a plurality of stock solutions, to processing units which are used to fabricate semiconductor devices. If a chemical supplied to the processing units is unstable due to changes in its composition, aggregation of finely divided particles contained in the chemicals, etc., the semiconductor devices will be defective. Accordingly, chemicals supplying apparatus which supply stable chemicals are required.
Conventional chemical supplying apparatus, for example, a slurry feeder which supplies a slurry to a chemical machine-polishing unit (hereinafter simply referred to as CMP unit) includes a first tank in which stock solutions are diluted and mixed to prepare the slurry and a second tank in which the slurry is stored. The slurry feeder first draws stock-solution (e.g., a suspension of alumina serving as abrasive grains and a solution of ferric nitrate serving as an oxidizing agent) from stock solution tanks and supplies the stock solutions to the first tank. The slurry feeder also supplies pure water to the first tank to carry out diluting and mixing treatment, thereby forming a slurry having a predetermined concentration. The slurry feeder then feeds the slurry to the second tank to store the slurry therein. The slurry feeder supplies the slurry to CMP units employing various kinds of pumps based on commands from the CMP units during polishing treatment. When the amount of slurry in the second tank decreases to a preset level, the slurry feeder prepares a new batch of slurry to supplement the slurry in the second tank, ensuring storage of a sufficient amount of slurry in the second tank.
Slurries tend to aggregate when dried or at sites where they dwell. Accordingly, aggregation of a slurry in a passage through which the slurry flows prevents the slurry feeder from supplying the slurry. Unfortunately apparatuses for feeding only general fluids, which do not have mechanisms for flushing passages through which slurries flow, have conventionally been utilized as slurry feeders. Accordingly, the slurry in the passage or pipe aggregates, causing clogging of the pipe. In addition, agglomerates of abrasive grains can be supplied to CMP units and form scratches on the surfaces of wafers undergoing polishing treatment, leading to low wafer yield.
Further, in slurries, particularly metal slurries prepared by mixing and diluting a suspension of alumina serving as abrasive grains and a solution of ferric nitrate serving as an oxidizing agent, precipitation occurs relatively quickly. Thus, polishing rates (speed etc.) decrease over. Such reduction in the polishing rates means that the thus formed slurry has a predetermined tank life. However, in the system where slurries are continuously stored in the second tank, former batches of slurries remain in the tank, which causes variations in the wafer polishing period, making it impossible to achieve high-accuracy polishing of wafers.
In the apparatus for supplying a chemical, since the chemical stored in the second tank evaporates, which changes concentrations of the components in the second tank, it is not preferred to store the chemical in the second tank over a long period. Accordingly, chemicals not used over long periods are frequently discarded, leading to waste of chemicals and stock solutions.
It is an objective of the present invention to provide an apparatus for supplying a chemical which can supply new batches of chemical solution stably.